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Mailcap files are used to configure how MIME-aware applications such as mail
readers and Web browsers react to files with different MIME types. (The name
“mailcap” is derived from the phrase “mail capability”.) For example, a mailcap
file might contain a line like video/mpeg;xmpeg%s. Then, if the user
encounters an email message or Web document with the MIME type
video/mpeg, %s will be replaced by a filename (usually one
belonging to a temporary file) and the xmpeg program can be
automatically started to view the file.

The mailcap format is documented in RFC 1524, “A User Agent Configuration
Mechanism For Multimedia Mail Format Information,” but is not an Internet
standard. However, mailcap files are supported on most Unix systems.

Return a 2-tuple; the first element is a string containing the command line to
be executed (which can be passed to os.system()), and the second element
is the mailcap entry for a given MIME type. If no matching MIME type can be
found, (None,None) is returned.

key is the name of the field desired, which represents the type of activity to
be performed; the default value is ‘view’, since in the most common case you
simply want to view the body of the MIME-typed data. Other possible values
might be ‘compose’ and ‘edit’, if you wanted to create a new body of the given
MIME type or alter the existing body data. See RFC 1524 for a complete list
of these fields.

filename is the filename to be substituted for %s in the command line; the
default value is '/dev/null' which is almost certainly not what you want, so
usually you’ll override it by specifying a filename.

plist can be a list containing named parameters; the default value is simply
an empty list. Each entry in the list must be a string containing the parameter
name, an equals sign ('='), and the parameter’s value. Mailcap entries can
contain named parameters like %{foo}, which will be replaced by the value
of the parameter named ‘foo’. For example, if the command line showpartial%{id}%{number}%{total} was in a mailcap file, and plist was set to
['id=1','number=2','total=3'], the resulting command line would be
'showpartial123'.

In a mailcap file, the “test” field can optionally be specified to test some
external condition (such as the machine architecture, or the window system in
use) to determine whether or not the mailcap line applies. findmatch()
will automatically check such conditions and skip the entry if the check fails.

Returns a dictionary mapping MIME types to a list of mailcap file entries. This
dictionary must be passed to the findmatch() function. An entry is stored
as a list of dictionaries, but it shouldn’t be necessary to know the details of
this representation.

The information is derived from all of the mailcap files found on the system.
Settings in the user’s mailcap file $HOME/.mailcap will override
settings in the system mailcap files /etc/mailcap,
/usr/etc/mailcap, and /usr/local/etc/mailcap.